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6. Long-Term Debt (Details) - USD ($)
$ in Thousands
Jun. 30, 2015
Dec. 31, 2014
Dec. 31, 2010
Debt Instrument [Line Items]      
Debt and Capital Lease Obligations $ 435,900 $ 436,000  
Less Current Maturities (268) (265)  
Long-term debt, net of current maturities 435,660 435,710  
Unsecured Debt [Member] | Senior Notes 4.875% [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Interest Rate, Stated Percentage     4.875%
Long-term debt 423,600 423,500 $ 422,600
Debt Instrument, Interest Rate, Effective Percentage     4.946%
Face amount of debt sold     $ 425,000
Capital Lease Obligations [Member] | Capital Leases and Other Debt [Member]      
Debt Instrument [Line Items]      
Long-term Debt and Capital Lease Obligations, Including Current Maturities 12,300 $ 12,500  
Line of Credit [Member]      
Debt Instrument [Line Items]      
Line of credit maximum borrowing capacity 200,000    
Outstanding borrowings $ 0    
Line of Credit Facility, Interest Rate at Period End 1.53%    
Standby Letters of Credit [Member]      
Debt Instrument [Line Items]      
Line of credit maximum borrowing capacity $ 5,000